Probable Molecular Weight Research Articles

Bioactive exopolysaccharide from Neopestalotiopsis sp. strain SKE15: Production, characterization and optimization.

Fungal exopolysaccharides are powerful resources of medicinal applications. Neopestalotiopsis sp. SKE15 was isolated and identified according to phenotypical and genotypical analyses (GenBank Accession No. MG649986). The exopolysaccharide (EPS) was produced by cultivation of mycelia in broth culture and extracted. The production was optimized to 2.02 g/l after selection of agitation, temperature, FeSO4 and K2HPO4 concentrations as the most influencing factors using Placket-Burman design and then by applying response surface methodology. Analytical Tools showed that the EPS is composed of a polysaccharide (1.5-2.1 × 106 Da) and its probable low molecular weight derivatives, in a wide range of chain lengths, among them an oligosaccharide of about 1970 Da was dominant. GC-MS (Gas chromatography-mass spectrometry) analysis revealed the EPS was mainly constructed from d-glucose, sorbitol and D-galactose. The EPS showed antibacterial activity against Staphylococcus aureus ATCC 25923, Bacillus subtilis ATCC 6633 and Pseudomonas aeruginosa ATCC 27853. DPPH (1,1-diphenyl-2-picryl-hydrazyl) and hydroxyl radical scavenging activity assays showed strong antioxidant activity of the EPS. A challenge with three different cancerous cell lines showed cytotoxic activity of the EPS at final concentration of 100 and 200 μg/ml. Further investigation on medicinal applications of the biopolymer is promising.

Suppression of Crystallization in Polydimethylsiloxanes and Chain Branching in Their Phenyl-Containing Copolymers

While incorporation of only a few mol % of diphenylsiloxy-, DiPhS, repeat units into polydimethylsiloxane, PDMS, chains is enough to completely suppress their crystallization, it also leads, in polymers obtained by silanolate-initiated ring-opening polymerization, to a puzzling and yet unexplained chain branching which significantly distorts polymer molecular weight distributions and affects their chain conformation. In contrast to this, we found that introduction of comparable amounts (ca. 5 mol %) of diethylsiloxy-, DiEtS, units into the same type of polymers also suppresses polymer crystallization but does not lead to any detectable branching, yielding polymers with the most probable molecular weight distribution and typical random coil conformation in a thermodynamically good solvent, such as toluene. On the basis of the results of a 29Si NMR and SEC-MALS study, we propose that branching in DiPhS-containing polymers is caused by a nucleophilic attack of initiating silanolate anions on their Si–CAr sid...

Cloning, characterization and expression of a novel haplotype cry2A-type gene from Bacillus thuringiensis strain SWK1, native to Himalayan valley Kashmir

Bacillus thuringiensis (Bt) is a gram positive bacterium which is effectively being used in pest management strategies as an eco-friendly bioinsecticide. In the present study a new cry2A gene was cloned from a promising indigenous B. thuringiensis SWK1 strain previously characterized for its toxicity against Spodoptera litura and Helicoverpa armigera larvae. The nucleotide sequence of the cloned cry2A gene pointed out that the open reading frame has 1902 bases encoding a polypeptide of 634 amino acid residues with a probable molecular weight of 70kDa. Homology comparisons showed that the deduced amino acid sequence of Cry2A had a similarity of 94% compared to that of the known Cry2Aa protein in the NCBI database and this gene has been named as cry2Al1 by the B. thuringiensis δ-endotoxin Nomenclature Committee. cry2Al1 was ligated into pET 22b vector and expressed in Escherichia coli BL21 (DE3) pLysS under the control of T7 promoter induced by isopropyl-beta-d-thiogalactopyranoside (IPTG). SDS-PAGE analysis confirmed the expression of cry2Al1 as ∼65kDa protein. Insect pest bioassays with neonate larvae of S. litura and H. armigera showed that the purified Cry2Al1 are toxic to S. litura and H. armigera with LC50 2.448μg/ml and H. armigera with 3.374μg/ml respectively.

Purification and Biochemical Characterization of Lipase from Tunisian Euphorbia peplus Latex

AbstractLipases from vegetable sources have been the focus of intense and growing research. The use of enzymes from plants has the advantage of employing industrial waste products. The lipase activity of Euphorbia peplus L. (Euphorbiaceae) was investigated for the first time. The Euphorbia peplus latex lipase (EpLL) was purified after ammonium sulfate fractionation and anion exchange chromatography on a DEAE‐Cellulose column leading to 12.57‐fold purification. The EpLL displayed a probable molecular weight of about 40 kDa. The lipase activity was optimum at a temperature of 40 °C and pH 8, the specific activities of EpLL were found to be 249 ± 12.45 and 161.4 ± 8.07 U/mg when tributyrin (TC4) and olive oil were used as substrate respectively. The enzyme retained 80 % of its activity when incubated for 1 h at 50 °C. The EpLL was strongly destabilised by divalent metal ions (Fe2+, Mg2+, Zn2+ and Cu2+). Lipase was slightly stimulated by Triton X‐100 and Tween‐80, while strongly inhibited by sodium dodecyl sulfate. A good stability of the enzyme in the presence of organic solvents was reveled suggesting its industrial utility.

Advances in Asphaltene Science and the Yen–Mullins Model

The Yen–Mullins model, also known as the modified Yen model, specifies the predominant molecular and colloidal structure of asphaltenes in crude oils and laboratory solvents and consists of the following: The most probable asphaltene molecular weight is ∼750 g/mol, with the island molecular architecture dominant. At sufficient concentration, asphaltene molecules form nanoaggregates with an aggregation number less than 10. At higher concentrations, nanoaggregates form clusters again with small aggregation numbers. The Yen–Mullins model is consistent with numerous molecular and colloidal studies employing a broad array of methodologies. Moreover, the Yen–Mullins model provides a foundation for the development of the first asphaltene equation of state for predicting asphaltene gradients in oil reservoirs, the Flory–Huggins–Zuo equation of state (FHZ EoS). In turn, the FHZ EoS has proven applicability in oil reservoirs containing condensates, black oils, and heavy oils. While the development of the Yen–Mullin...

Gel growth in free radical crosslinking copolymerization: Effect of inactive gel radicals

A kinetic model is presented for the post-gelation period of free-radical crosslinking copolymerization. The model takes into account the trapped radical centers in the gel forming system. It was shown that the weight fraction of sol, Ws, relates to the number of crosslinked units per weight-average primary molecule, ε, through the equation where n = 2 for Flory's most probable molecular weight distribution, and n = 3 for primary molecules formed by radical combination. Calculation results demonstrate that the existence of trapped radicals significantly affects the growth rate of the gel molecule. It increases the total radical concentration and accelerates the gel growth. The difference in the predictions with and without considering the trapped radicals becomes significant as the crosslinker concentration decreases or, as the vinyl group reactivity on the crosslinker or on the polymer decreases.

The effect of molecular weight and crystallite structure on yielding in ethylene copolymers

Nominal stress-strain curves of a series of random ethylene-hexene copolymers having narrow composition and most probable molecular weight distributions were investigated. A series of such molecular weight copolymers with a constant concentration of branches were crystallized under a variety of conditions. In each molecular weight series the level of crystallinity was approximately constant. Particular attention was focused on the yield region and the nature of the yielding process. It was found, quite surprisingly, that the yield stress was not solely dependent on the crystallinity level. Moreover, the shape of the force-elongation curve in the yield region was very dependent on the molecular weight and the crystallization mode. These changes in yielding correlated quite well with the overall crystallite structure that was characterized by thin section transmission electron microscopy. The orthorhombic unit cell of polyethylene was maintained in all the samples despite the changes that occurred in the overall crystallite structure. © 1997 John Wiley & Sons, Inc.

Solutions of the general equations for the radiation dose dependence of the molecular weights of irradiated polymers with an initial Schulz-Zimm distribution

The general equations for the radiation dose dependence of irradiated polymer molecular weights have been solved exactly. For an initial most probable molecular weight distribution (σ=1), the solutions are analytical and exact. For the general case (σ≠1) the solutions are numerical and exact. The present approach has resulted in the solutions for both σ=1 and σ≠1 being incorporated into a group of FORTRAN computer programs which will solve experimental data for scission and crosslinking yields by both minimization and exact treatments. Simulated data treated using these FORTRAN programs are give. The FORTRAN programs are available from the authors.

Tensile Properties of Crystalline Polymers: Random Copolymers of Ethylene

Force-elongation curves of a set of random ethylene-1-alkene copolymers have been studied. The comonomers included 1-butene, 1-hexene, 1-octene, and 4-methyl-1-pentene. The copolymers all had the most probable molecular weight and narrow composition distributions. A set of hydrogenated poly(butadienes), random ethyl-branched copolymers, that have very narrow molecular weight and composition distributions were also studied as reference. Only ductile type deformations were studied. Several important generalizations resulted from this work. An important finding was that the molecular weight, copolymer composition, and chemical nature of the co-units have to be considered as independent variables in analyzing the tensile behavior. The nominal stress-strain curves of even modest molecular weights are dominated by strain-hardening. This characteristic is similar to that of very high molecular weight linear polyethylenes. However, the effect is not as large for the ethylene-butenes as compared with the other copolymers. The ultimate properties of the ethylene-butene copolymers also differ from the others. An explanation for these differences can be given by postulating that in the melt structure, and thus in the residual liquid-like region, noncrystalline portions are affected by interaction of the side groups. In contrast, the yield stress and initial modulus do not depend on the chemical nature of the co-unit but only on the crystallinity level. In the regions of overlap there is a substantial difference between the yield stress and initial modulus of linear polyethylene and the copolymers. Different portions of the force-elongation curves are governed by different structural and molecular features indicating the complexity of the problem. The wide range of experimental data that are presented, for thoroughly characterized samples, can serve as a basis for the molecular understanding of the ductile deformation of random copolymers.

Metathesis polymerization — stepwise or chain growth reaction?

The metathesis polymerization of cycloolefins may follow a stepwise or a chain growth mechanism depending on the type of double bond involved (strained or unstrained) and the catalyst (high or low activity). Strained cycloolefins/catalysts of low activity/catalysts with low transfer ability, in the kinetically controlled regime of the polymerization, yield high molecular weight polymers following a chain growth mechanism; in the ideal case, the molecular weight distribution of the polymers is identical with a Poisson distribution. Under suitable conditions, equilibrium may be attained during the course of the reaction, which yields cyclic oligomers corresponding to a ring-chain equilibrium and a most probable distribution of the polymers. On the other hand, with cycloolefins of low ring strain/highly active catalysts, the reaction yields cyclic oligomers in the initial steps of the polymerization, the dimers and trimers being particularly preferred and produced in greater amounts than those corresponding to their equilibrium concentration. In the further course of the reaction they are consumed in a step-wise manner, to form high molecular weight material exhibiting a most probable molecular weight distribution and to result in a homologous series of cyclic oligomers corresponding to a ring-chain equilibrium.

Mechanism of anionic ring‐opening polymerization of silacyclopent‐3‐enes

AbstractAnionic ring‐opening polymerization of silacyclopent‐3‐enes has characteristics which are different from those of anionic living polymerizations of 1,3‐dienes. High molcular weights are obtained in most cases but generally with most probable molecular weight distributions Mw/Mn ≅ 2. The polymerization is stereospecific. Poly(1‐sila‐cis‐pent‐3‐enes) are obtained. Ring opening polymerization of 1,1,3‐trimethyl‐1‐sila cyclopent‐3‐ene is found to be regioselective at −78°C to yield a polymer in which cis‐1,4‐isoprene units are joined to dimethylsilylene units almost exclusively in a head to tail orientation. Finally, anionic polymerization of 1‐methyl‐1‐silacyclopent‐3‐ene is found to yield poly(1‐methyl‐1‐sila‐cis‐pent‐3‐ene) of low molecular weight. This permits end group analysis by 1H, 13C and 29Si NMR spectroscopy, signals are observed which are consistent with the presence of 1‐methyl‐1‐silacyclopent‐3‐ene rings. These are probably end groups. A mechanism consistent with the above is discussed.

Gel/dose curves for polymers undergoing simultaneous cross-linking and scission

This paper analyses the effect of molecular weight distribution on the relation between sol fraction s and absorbed dose D of ionising radiation. The usual Charlesby-Pinner relation with random cross-linking and scission was derived for the most probable initial molecular weight distribution ( M w: M n = 2 ), but is frequently used for very different distribution, to which it is not applicable. A modified version of this relation is presented for such other distribution, involving a virtual additional dose D v . Its success is assessed by comparing it with published experimental data for a number of polymers. The paper also compares these data with the empirical Zhang-Wanxi equation.

Gel/dose curves for polymers undergoing simultaneous crosslinking and scission

This paper analyses the effect of molecular weight distribution on the relation between sol fraction s and absorbed dose D of ionising radiation. The usual Charlesby-Pinner relation with random crosslinking and scission was derived for the most probable initial molecular weight distribution ( M w : M n =2) but is frequently used for very different distribution, to which it is not applicable. A modified version of this relation is presented for such other distribution, involving a virtual additional dose D v . Its success is assessed by comparing it with published experimental data for a number of polymers. The paper also compares these data with the empirical Zhang equation.

Turkey gizzard caldesmon: molecular weight determination and calmodulin binding studies.

Sedimentation equilibrium and sedimentation velocity measurements demonstrate that turkey gizzard caldesmon is an elongated molecule of molecular mass 75 +/- 2 kDa. The frictional ratio (2.14) is consistent with a prolate ellipsoid of axial ratio 24, corresponding to an apparent length and width of 516 and 21.5 A, respectively. As was previously determined for chicken gizzard caldesmon [Graceffa, P., Wang, C.-L.A., & Stafford, W.F. (1988) J. Biol. Chem. 263, 14196-14202], this molecular weight is appreciably smaller than the value (approximately 135,000) estimated from the results of NaDodSO4 gel electrophoresis experiments. However, a significant difference between the true molecular weights of turkey and chicken gizzard caldesmons--75,000 versus 93,000--also points to probable molecular weight variations within the subclass. Binding measurements, based on perturbation of the intrinsic tryptophan fluorescence of caldesmon in the presence of calmodulin, show that the interaction between the two proteins is strongly ionic strength and temperature dependent. Dissociation constants of 0.075 and 0.38 microM were determined in solutions containing 0.1 and 0.2 M KCl, respectively, at 24.3 degrees C. Fluorescence emission spectra and fluorescence anisotropy excitation spectra indicate that the tryptophanyl residues of caldesmon are located in solvent-accessible regions of the molecule, where they exhibit a high degree of mobility even when calmodulin is bound.

Characterization of Polymers by GPC-LALLS. III. Branching Structure of EPDM

Abstract Gamma ray-irradiated EPM's, as the model polymers of branched EPDM, are investigated using the relationship g′=gb, where g′ is the ratio of intrinsic viscosities of the branched and the linear molecules of equal molecular weight, [η]br/[η]l, and g, the ratio of the mean square radii of gyration of the two, 〈s2〉br/〈s2〉l. The molecular weight distributions measured by GPC-LALLS coincide well with theoretical curves of tetrafunctionally and statistically branched polymers obtained by the ideal degradation and crosslinking of the raw EPM, which was assumed to have the most probable molecular weight distribution, and the b-value is then determined to be 1.1. EPDM samples, polymerized with a soluble vanadium compound—alkyl aluminum halide type catalyst in a continuous well-stirred pilot-reactor, are characterizedas to the number of branching points per molecule for various molecular weights by using the b-value. The higher the molecular weight, the smaller the distance between neighboring crosslinking points. The reason is discussed. The unsaturated bond of dicyclopentadiene crosslinks more readily in the manufacturing process than 5-ethylidene-2-norbornene. The largest high molecular weight portion and the broadest molecular weight distribution are observed in the EPDM with the maximum dicyclopentadiene content.

Low temperature radiation-induced polymerization of 2-hydroxyethyl methacrylate for polymer matrix preparation

Radiation-induced polymerizations of carefully purified 2-hydroxyethyl methacrylate in the presence of methanol in various proportions were carried out at −78°C. The polymerization kinetics were found to be markedly influenced by the solvent amount and roughly to follow a first order law with respect to the monomer. Viscometric and light-scattering measurements were carried out on poly(2-hydroxyethyl methacrylate) samples dissolved in 2-methoxyethanol; the relationship between [η] and M ̄ w was established and found to agree with results reported by other authors with polymer samples prepared at normal temperatures. By assuming the most probable molecular weight distribution, the chain transfer constant to the solvent was calculated. The porosity of polymer matrices obtained from monomer-water mixtures was also investigated using optical microscopy.

The metathesis reaction of cycloolefins and heterocyclic olefins: Thermodynamics and mechanistic aspects

The metathesis reaction of cycloolefins results in polymers consisting of open chain molecules showing a most probable molecular weight distribution. Beside the polymer residual monomer and/or a homologous series of cyclic oligomers are formed by a “back-biting” reaction. The relation between acyclic polymer and cyclic oligomers can be aiscussed in terms of a ring chain equilibrium. The equilibrium, however, is not established instantaneously; the reaction may be kinetically hindered. The oligoner concentration in the kinetically controlled regime is proportional to x−3/2when the thermodynamic equilibrium is established the oligomer distribution is determined by a ring chain equilibrium. From the Jacobson-Stockmayer plots a characteristic ratio of 5,8 is derived for polyoctenylene and of 13,9 for polynorbornene. The Casey carbene (CO)5W=C(C6H5)2 is more active as catalyst than carbenes of the Fischer type such as (CO)5W=C(OCH3)C6H5; Fischer carbenes are only active with respect to strained olefins. Consequently, in the presence of the latter carbene only polymers are observed; the “back-biting” reaction to yield the cyclic oligomers does not occur. The corresponding chromium carbenes (CO)5Cr=C(R)C6H5 promote the metathesis reaction of vinylethers. Cyclic vinylethers, e.g. 2,3-dihydrofuran and 2,3,4,5-tetrahydrooxepin-2-yl acetate are polymerized in a fully head-to-tail fashion.

Olefin metathesis-distribution of products and carbene complex initiation

The metathesis reactions of cycloolefins results in a ring-chain equilibrium of cyclic oligomers and an acyclic polymer. The distribution of the polymer can be described by a most probable molecular weight distribution. The oligomer distribution differs in the kinetically controlled regime of the reaction from the equilibrium situation. The particular cases of norbornene and norbornadiene are discussed. Carbene complexes may be used as initiators. The more stable the carbene the more strained the cycloolefin has to be in order to be polymerizable. With increasing strain the cis-stereoselectivity is significantly attenuated. (CO)5CRCR′R″ may initiate the metathesis reaction of cyclic and acyclic vinylethers. The reaction proceeds in a structurally uniform manner, i.e. only vinylethers are formed, and yields the equilibrium ratio of cis and trans double bonds.

On the morphology of aromatic polyamide fibers (Kevlar, Kevlar-49, and PRD-49)

Abstract Three types of aromatic polyamide fibers (Kevlar, Kevlar-49, and PRD-49) were studied by transmission electron microscopy, secondary electron imaging, and electron diffraction techniques. Ultramicrotomy, ion-thinning, and brittle fracture techniques were used to prepare specimens which revealed both longitudinal and transverse fiber structures. Aromatic polyamide fibers exhibit both a “skin” and a “core” structure. The core morphology is shown to be consistent with a layered structure where individual layers are stacked perpendicular to the fiber axis and are composed of rodlike crystallites whose c axes are parallel to the fiber axis. Electron diffraction results indicate that hydrogen bonds are located nearly in the radial direction of the fiber. The crystallite layer thickness appears to depend upon the most probable molecular weight of the fiber. Longer crystallites which pass through two or more layers are present, and may control the strength of the fiber in the axial direction.

The preparation and characterization of different types of light-harvesting pigment-protein complexes from some purple bacteria.

A general strategy, with some specific examples, is given for the isolation and purification of detergent-soluble, antenna pigment-protein complexes from the photosynthetic membranes. Absorption, fluorescence and circular dichroism spectra, and the pigment and protein composition of B800-B850-protein and B890-protein complexes of some purple bacteria (Rhodospirillum rubrum, Rhodopseudomonas sphaeroides and Rps. capsulata and Chromatium vinosum) are discussed. We conclude that there are probably two major classes of antenna carotenochlorophyll-proteins in purple bacteria containing bacteriochlorophyll a: a B890 complex which has one carotenoid and two bacteriochlorophyll molecules in the minimal unit (probable molecular weight around 20,000), and a B800 + B850 complex which has one carotenoid and three bacteriochlorophyll molecules in a similar-sized minimal unit. The whole cell spectrum of any purple bacterium can be reconstituted by combining different proportions of the spectra of these two complexes with that of the photochemical reaction centre.